Monostable multivibrator utilizing unidirectional device in cross-coupling to isolate output from input during quiescent condition



Dec. 28, 1965 F. L. ROGERS; JR 3,226,576

MONOSTABLE MULTIVIBRATOR UTILIZING UNIDIRECTIONAL DEVICE INCROSS-COUPLING TO ISOLATE OUTPUT FROM INPUT DURING QUIESCENT CONDITIONOriginal Filed Sept. '7, 1961 FIG.1

INPUT 1 BASE 18 ATTORNEY.

United States Patent MONOSTABLE MULTIVIBRATOR UTILIZING UNI- DIRECTIONALDEVICE IN CRGSS-CGUPLING T0 ISOLATE UUTPUT FROM INPUT DURING QUIESCENTCONDITION Fred L. Rogers, Jr., Lexington, Ky., assignor to InternationalBusiness Machines Corporation, Armonk, N.Y., a corporation of New YorkContinuation of application Ser. No. 136,473, Sept. 7, 1961. Thisapplication Nov. 18, 1964, Ser. No. 415,846

4 Claims. (Cl. 3tl788.5)

This is a continuation of application Serial No. 136,473, filedSeptember 7, 1961, now abandoned.

This invention relates to an electrical amplifier circuit and moreparticularly to a circuit effective to produce a potential output pulseof a predetermined constant time duration, in response to a triggerinput pulse and irrespective of applied input potentials following thetrigger input pulse.

In certain applications, it is desirable or even necessary to produce apotential pulse of a certain Width in response to an applied potentialof at least a certain magnitude. It therefore follows that the circuitproducing the potential pulse must be insensitive to applied inputpotentials once the production of the desired output pulse is initiated.

A well-known circuit for producing a desired output pulse of apredetermined width in response to the application of a potential of acertain magnitude is the monostable multivibrator. In this circuit apair of active circuit devices, which may be electron tubes ortransistors, have respective input and output elements cross coupled andare biased in such a manner that in a quiescent condition of the circuitone of the devices, is in a state of high conduction and the other ofthe devices is in a state of very low conduction. The cross couplingbetween these devices has a time constant circuit which may include aresistor and a capacitor which is charged through the resistor. Thecircuit may be triggered by a pulse applied to an appropriate node ofthe circuit so as to either charge or discharge the capacitor throughthe resistor of the time constant circuit and this period of timedetermines the transient condition of the circuit. During this transientcondition the transistor previously in a state of high conduction is ina state of low conduction and the transistor previously in a state oflow conduction is in a state of high conduction. On the completion of apredetermined amount of either charge or discharge of the capacitor inthe time constant circuit the multivibrator circuit revertsautomatically to its quiescent condition and is thus ready to receiveanother input pulse to undergo another transition to its transientcondition and reversion to its quiescent condition.

While this circuit has been applied with admirable success for thepurpose of producing output pulses of a predetermined duration inresponse to the application of triggering pulses of a predeterminedmagnitude the same have been susceptible to extraneous influences suchas noise which may occur in the circuitry involved for interrupting orshortening the transient period of the circuit. As a consequencecircuits following the monostable multivibrator and relying upon themultivibrator for reliable pulses of a predetermined time duration areimproperly excited resulting in erroneous operation or no operation atall.

In the usual multivibrator circuit of the type herein described, theinput active circuit device is maintained in either a state of highconduction or low conduction by a biased potential derived from theoutput of another circuit device. In the usual case this biasingpotential is of considerably greater magnitude than is required for thepurpose of maintaining the input active circuit device ice in itsquiescent condition. Thus an input trigger pulse of considerablemagnitude is required to trigger the circuit into its transientcondition. While in most cases this is not an adverse feature sincepulses of considerable magnitude are available, in certain applicationsthe input pulses received may desirably be of relatively smallmagnitude. In this case it would be undesirable to require the inputpulse to surmount the magnitude of the biasing pulse from the outputcircuit of the other active circuit device for triggering the circuitinto its transient condition.

It is accordingly a principal object of this invention to provide amonostable multivibrator circuit which is responsive to input pulses ofrelatively small magnitude and which further is insensitive toextraneous potentials produced, as by noise, for producing an outputpotential of a predetermined pulse width in response to application ofinput pulses to the circuit.

In accordance with this invention the output potential of amultivibrator circuit utilized for biasing the input active circuitdevice to its quiescent condition is isolated therefrom by means of aunidirectional circuit element after the circuit has made its transitionfrom its transient condition to its quiescent condition. Thus, theactive circuit element at the input of the circuit is influenced only bythe input trigger pulses and no longer by the output potential of theother active circuit device in the multivibrator circuit.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

FIGURE 1 is a schematism of monostable multivibrator utilizing featuresof the present invention, and

FIGURE 2 shows wave forms of potentials at certain nodes of the circuitshown in FIGURE 1.

Referring now to the drawings for a more detailed description of theinvention, in FIGURE 1, 10 represents generally a monostablemultivibrator circuit which includes as active circuit devicesrespective transistors 12 and 14 which are shown as being of the NPNjunction type. Transistor 12 has an emitter 16, base 18, and collector20; and transistor 14 includes an emitter 22, base 24, and collector 26.The collector 20 of transistor 12 is coupled to base 24 of transistor 14through a capacitor 28 and the collector 26 of transistor 14 is coupledto the base 18 of transistor 12 through a resistor 30 and a seriallyconnected unidirectional diode 32 having its anode connected directly tobase 18 and its cathode connected to resistor 30. Base 18, collector 2d,and collector 26 are all positively biased from a source of potentialpositive with respect to ground through respective resistors 34, 36, and38 connected between this source of potential and these respectiveelements. The junction between resistor 30 and diode 32 and the base 24of transistor 14 are negatively biased by a source of potential negativewith respect to ground connected to these respective points throughrespective resistors 4t and 42. The emitters 16 and 22 are eachconnected to ground as shown. For the purpose of applying inputpotential pulses to the monostable multivibrator circuit an inputterminal 44 is provided and is connected to base 13 through a capacitor46 in series with a resistor 48. Output potentials from the circuit 10are derived from an output terminal 50 connected directly to collector26.

In the absence of input potentials applied at input terminal 44 and inresponse to the application of potentials represented by the symbols |Vand V, the circuit assumes a quiescent condition in which transistor 12is in a state of saturation, or in other words, heavy conduction andtransistor 14 is in a state of very low conduction substantially atcutofi". It is noted that this condition results from the forward biasestablished at the junction between base 18 and emitter 16 by thepositive potential applied to base 18 through resistor 34 and thenegative potential applied to base 24 from the source of potential, V,through the resistor 42. It is significant to note that the potential atthe cathode of diode 32, which is at a point of a potential dividerestablished by resistors 38, 30, and 40 across the sources of potential+V and V, is positive with respect to ground by reason of theproportioning of resistor 40 relative to resistor 38 and 30. Thus, thediode 32 is reverse biased.

For an understanding of the circuit 10 shown in FIG- URE 1 of thedrawings reference is also made to FIG- URE 2 of the drawings showingwave forms of potentials at certain nodes of this circuit. A typicalinput potential is represented by the wave form 52. This potential maybe supplied, for example, by a magnetic reading head which reads a bitof information fro-m a magnetic tape. As shown, this potential decreasesfrom a time t to a point 54 at time t1. At this time the potential atbase 18 of transistor 12 is reduced to the point that the base-emittedjunction of this transistor is reverse biased and conduction in thetransistor is interrupted. The potential at base 18 is represented byWave form 56 and as shown, this potential drops abruptly fromsubstantially zero volts to a very low value. This high reverse biaspotential is supplied by the potential sourse V through the voltagedivider circuit of resistors 40 and 30. Since the base-emitter currentis intrrupted, the emitter-collector current is also interrupted in thistransistor whereby the collector potential suddenly rises to a value ofsubstantially -+V. This is represented by the wave form 58 in FIGURE 2.

The collector 20 of transistor 12 is coupled to the base 24 oftransistor 14 through capacitor 28 whereby the positive pulseestablished at collector 20 is applied to the base 24. The result ofthis application ofthis potential pulse is to bias the base-emitterjunction of transistor 14 in a forward direction whereby the currenttherein suddenly rises to a very high value. This is represented by thewave form 60 in FIGURE 2 of the drawings. The sudden rise of current intransistor 14 produces a negative pulse at its collector and thisnegative pulse is applied through resistor 30 and diode 32 to the base18 of transistor 12. The negative pulse appearing at the collector 26 oftransistor 14 is represented at 62 in FIG- URE 2 of the drawings. Duringthe transient interval of time from t1 to t3, the base 18 of transistor12 is biased negatively from the potential source, V, applied throughresistor 40 and diode 32 to maintain this transistor insensitive tospurious potentials that may be applied to input terminal 44. It is thusobserved that the action initiated by the potential Wave form 52 isregenerative and thus transistor 12 assumes a state of very lowconduction near cutoff and transistor 14 assumes a condition of currentsaturation. This is the transient condition of the circuit 10. Duringthe transient condition, capacitor 28 is charged through a seriescircuit including the base-emitter junction of transistor 14, thecapacitor 28 itself, and resistor 36. The voltages produced during thecharging of the capacitor 28 are shown on wave form 60. It will be notedthat the high surge between times 1 to t2 saturates the components inthe preferred embodiment shown, but that the well known exponentialcurve then appears to provide biasing current until the potential atbase 24 drops to zero. At time t3 the baseemitter junction of transistor14 becomes reversely biased and current flow in this transistor becomesinterrupted. The sudden drop in potential at base 24 is represented onwave form 60 at time t3. The negative potential feed back from collector26 to base 18 of transistor 12 is also interrupted. Thus, the influenceof potential source 4 +V is to apply a positive potential to the base 18of transistor 12 through resistor 34. Accordingly, transistor 12 revertsto a state of saturation conduction. This, again, is the quiescent stateof the circuit 10.

In the quiescent condition of circuit 10, resistors 34 and 48 cooperatewith transistor 12 and potential source +V to provide a discriminatorcircuit effective ,to minimize the adverse effects of noise introducedon line 44. To this end, the base-emitter junction of transistor 12conducts .a predetermined value of current and the base 18' issubstantially at ground potential. Under these circumstances, resistor34 is proportioned with respect to potential +V, to supply a value ofcurrent considerably in excess of this predetermined value. Thus noisesignals introduced on line 44 which produce current conduction intocapacitor 46 must reach a value so as to cause a conduction of currentthrough resistor 48 greater than the value of excess current thatresistor 34 and potential, +V, can provide over the value of currentconducted through the base-emitter junction of transistor 12. Purely asan example, the base-emitter junction may conduct a current of the ordermioroamperes and of the order of 800 microamperes can be supplied fromsource, +V, through resistor 34. Thus the difference, or in other wordsapproximately 660 microamperes, can flow through resistor 48 before theconduction in the base-emitter junction of transistor 12 can besignificantly affected to change the condition of conduction in thistransistor. Stated otherwise, the input noise pulses can be absorbed ordiscriminated against to the extent that they cause conduction of 660microamperes through resistor 48.

In accordance with a feature of this invention provision is made so thatthe input signal pulses as represented on wave form 52 need onlysurmount this discriminator bias current in order to trigger the circuit10. That is to say, the potential appearing at collector 26 in thequiescent condition of the circuit 10 is not applied to base 18 sincethis base is isolated from this collector by reason of the reverse biason diode 32 in these circumstances. This is significantly different fromthe usual case of monostable multivibrators wherein the input pulse mustbe great enough to overcome this collector potential bias.

From the foregoing description it is apparent that in accordance withthis invention a monostable multivibrator circuit is provided which issubstantially immune from the adverse effects of noise on the input lineand which is effective for producing an output pulse of a predeterminedwidth in response to input signal potentials of at least a predeterminedmagnitude.

While the invention has been described with respect to a circuitutilizing NPN junction type transistors as active circuit devices, it isto be noted that it is equally applicable to circuits utilizing othertypes of transistors or even electron tubes as active circuit devices.Of course, in each situation appropriate changes of potential polarityand orientation of diode 32 must be made as will be readily understoodby one skilled in the art.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A circuit to produce an output pulse of predetermined time durationcomprising:

a first active element having an input and an output,

and connected to be normally in a first state of conduction,

means to connect a source of input pulses to the input of said firstactive element so that said first active element will be biased to asecond state of conduction by each of said pulses,

a voltage divider circuit having a first voltage division branch withtwo terminals and a second voltage division branch with two terminals,one of said terminals of said first voltage division branch and one ofmeans to connect a source of input pulses to the base of said firsttransistor so that said first transistor is biased to a second state ofconduction by each of said pulses,

said terminals of said second voltage division branch 5 a first circuitcomprising a resistor and having two ends, being electrically connected,the connection of said a second circuit comprising a resistor and havingtwo two voltage division branches defining a voltage diviends, sionpoint and the terminals of said two voltage divia voltage source poledto bias said first transistor to said sion branches electrically awayfrom said connection second state of conduction connected to one of saiddefining a first end and a Second end of Said Voltage 10 ends of saidfirst circuit comprising a resistor, the divider circuit, Said Voltagedivision Point being @011- other of said ends of said first circuitcomprising a hected to the ihPht of Said first active element, resistorbeing connected to one of said ends of said a second active elementhaving an input and an output, second circuit comprising a resistor,

the Output of Said Second active element being a second transistorhaving an emitter, a base, and a hectfid t0 the Said 611d of saidVoltage divider Circuit collector, the collector of said secondtransistor being terminating Said fil'st Voltagfi division anch, saidconnected to the remaining one of said ends of said Second activeelement being Connected to normally second circuit comprisinga-resistor, said second trancohduct at one State of conduction and toconduct sistor being connected to normally conduct at one a l h t stateof Conduction in response to a state of conduction and to conduct in achanged state slgna at Its mput of conduction in res onse to a si nal atits base, the a unidirectional Eonnccted in Series circuit with emitterof said secong transistor b eing connected to a said first voltage dv1s1on branch between the output predetermined plane of potential ofSald .Second active i ph the Input OI t means connecting said one end ofsaid second circuit first actlve element Sald untdlrectlonal .devlcebemg comprising a resistor to the base of said first transispoled to benormally back biased and being normally t back biased when said secondactive element is in 9 its normal state of conduction, a diode connectedin series circuit wlth said second C11- voltage source connected to thesaid end of said f compnsmg reslstol' between the Fonectot of voltagedivider circuit terminating said second voltt secohd tfahslstot and thebase of Sald first tranage division branch poled to bias said firstactive slstor, Sald dlode belhg Polfid to normally batfk element to saidsecond state of conduction, biased and being normally back blased When531d a circuit including an electrical energy storing element secondtransistor is in its normal state of conduction, for establishing a timeconstant connecting the outa circuit including a capacitor forestablishing a time P of said first active element and the input of Saidconstant connecting the collector of said first transis- Secohd activeelement, tor to the base of said second transistor, said first activeelement being connected to Produce a said first transistor beingconnected to produce a signal Signal when in Said second state ofconduction which when in said second state of conduction which will willPass Said circuit for establishing a time P pass said circuit forestablishing a time constant for a for a predetermined time supstantiauyequal i predetermined time substantially equal to said prepredeterminedoutput pulse time duration to bias said determined Output pulse timeduration to bias Said Sficond actlye element to a charged State offiend? second transistor to a changed state of conduction at at whlchthe Output of 8? 1d second actl-ve e which the output of said secondtransistor changes ment changes to a value at which a forward bias willf d b t d be presented to said unidirectional device to posito a Y atWhlch OrWar ms 2' e e tively bias said first active element againstchange to sftld dlode t Posltlvely blas first translstor in State ofconduction. against change in state of conduction. 2. The circuit as inclaim 1 in which said unidirec- Tlle C r t as m clalm 3 whlch sald diode1s tional device is connected betwen the input of said first connectedbetween the base of said first transistor and active element and saidvoltage division point. the point of connection of said other end ofsaid first 3. A circuit to produce an output pulse of predetercircuitcomprising a resistor and said one end of said sec- In n time durationcomprising? b d 1 0nd circuit comprising a resistor.

a first transistor having an emitter, a ase, an a co lector, andconnected to be normally in a first stgte No references cited. ofconduction, the emitter of said first transistor e- I ing connected to apredetermined plane of potential, R H GAUSS, Primary Exammer-

1. A CIRCUIT TO PRODUCE AN OUTPUT PULSE OF PREDETERMINED TIME DURATIONCOMPRISING: A FIRST ACTIVE ELEMENT HAVING AN INPUT AND AN OUTPUT, ANDCONNECTED TO BE NORMALLY IN A FIRST STATE OF CONDUCTION, MEANS TOCONNECT A SOURCE OF INPUT PULSES TO THE INPUT OF SAID FIRST ACTIVEELEMENT SO THAT SAID FIRST ACTIVE ELEMENT WILL BE BIASED TO A SECONDSTATE OF CONDUCTION BY EACH OF SAID PULSES, A VOLTAGE DIVIDER CIRCUITHAVING A FIRST VOLTAGE DIVISION BRANCH WITH TWO TERMINALS AND A SECONDVOLTAGE DIVISION BRANCH WITH TWO TERMINALS, ONE OF SAID TERMINALS OFSAID FIRST VOLTAGE DIVISION BRANCH AND ONE OF SAID TERMINALS OF SAIDSECOND VOLTAGE DIVISION BRANCH BEING ELECTRICALLY CONNECTED, THECONNECTION OF SAID TWO VOLTAGE DIVISION BRANCHES DEFINING A VOLTAGEDIVISION POINT AND THE TERMINALS OF SAID TWO VOLTAGE DIVISION BRANCHESELECTRICALLY AWAY FROM SAID CONNECTION DEFINING A FIRST END AND A SECONDEND OF SAID VOLTAGE DIVIDER CIRCUIT, SAID VOLTAGE DI VISION POINT BEINGCONNECTED TO THE INPUT OF SAID FIRST ACTIVE ELEMENT, A SECOND ACTIVEELEMENT HAVING AN INPUT AND AN OUTPUT, THE OUTPUT OF SAID SECOND ACTIVEELEMENT BEING CONNECTED TO THE SAID END OF SAID VOLTAGE DIVIDER, CIRCUITTERMINATING SAID FIRST VOLTAGE DIVISION BRANCH, SAID SECOND ACTIVEELEMENT BEING CONNECTED TO NORNALLY CONDUCT AT ONE STATE OF CONDUCTIONAND TO CONDUCT IN A CHANGED STATE OF CONDUCTION IN RESPONSE TO A SIGNALAT ITS INPUT, A UNIDIRECTIONAL DEVICE CONNECTED IN SERIES CIRCUIT WITHSAID FIRST VOLTAGE DIVISION BRANCH BETWEEN THE OUTPUT OF SAID SECONDACTIVE ELEMENT AND THE INPUT OF SAID FIRST ACTIVE ELEMENT, SAIDUNIDIRECTIONAL DEVICE BEING POLED TO BE NORMALLY BACK BIASED AND BEINGNORMALLY BACK BIASED WHEN SAID SECOND ACTIVE ELEMENT IS IN ITS NORMALSTATE OF CONDUCTION, A VOLTAGE SOURCE CONNECTED TO THE SAID END OF SAIDVOLTAGE DIVIDER CIRCUIT TERMINATING SAID SECOND VOLTAGE DIVISION BRANCHPOLED TO BIAS SAID FIRST ACTIVE ELEMENT TO SAID SECOND STATE OFCONDUCTION, A CIRCUIT INCLUDING AN ELECTRICAL ENERGY STORING ELEMENT FORESTABLISHING A TIME CONSTANT CONNECTING THE OUTPUT OF SAID FIRST ACTIVEELEMENT AND THE INPUT OF SAID SECOND ACTIVE ELEMENT, SAID FIRST ACTIVEELEMENT BEING CONNECTED TO PRODUCE A SAID FIRST ACTIVE ELEMENT BEINGCONNECTED TO PRODUCE A SIGNAL WHEN IN SAID SECOND STATE OF CONDUCTIONWHICH WILL PASS SAID CIRCUIT FOR ESTABLISHING A TIME CONSTANT FOR APREDETERMINED OUTPUT PULSE TIME DURATION TO BIAS SAID SECOND ACTIVEELEMENT TO A CHANGED STATE OF CONDUCTION AT WHICH THE OUTPUT OF SAIDSECOND ACTIVE ELEMENT CHANGES TO A VALUE AT WHICH A FORWARD BIAS WILL BEPRESENTED TO SAID UNDIRECTIONAL DEVICE TO POSITIVELY BIAS SAID FIRSTACTIVE ELEMENT AGAINST CHANGE IN STATE OF CONDUCTION.